The present invention relates to windmills. It is directed particularly to windmill support structures.
The towers that support windmills used for electric power generation are subject to considerable stresses that arise from the force of the wind on the windmill turbine. The wind force strains the tower, tending to bend it. The bending occurs to accommodate horizontal translation of the windmill and windmill pivoting. The windmill tower must therefore be designed not only to support the windmill against gravitation but also to withstand expected wind forces and the resultant bending in whatever mode it occurs.
It has been found that multi-leg towers are well suited to use with windmills. For a given amount of structural material, the strength of a multi-leg tower in resisting the wind force is relatively high. Moreover, the multi-leg tower is an open structure, so its interference with wind flow is low. And, because of the distribution of gravitational force through its structure, its foundation does not need to be very elaborate. In addition, it can be made easy to climb to provide access to the windmill for maintenance and repair.
However, the multi-leg tower can be subject to certain design complications that result from the periodic nature of the expected stresses. Even though the tower causes relatively little interference with wind flow, it still presents some. A windmill blade passing the tower therefore experiences a momentary reduction in wind force, and the windmill thus experiences a vibrational component having a fundamental frequency equal to the rotational frequency of the turbine multiplied by the number of turbine blades. Additionally, due to unavoidable asymmetries, a component of vibration can also be introduced at the rotational frequency of the turbine.
This expected vibration adds a constraint to the tower design. If the natural frequency of any bending mode of the tower is roughly equal to an expected vibrational frequency or one of its harmonics, the vibration can couple a significant proportion of wind power into the tower structure itself. Therefore, either the designer must provide the necessary strength--with the attendant material weight--required to withstand the vibration at the natural frequency of the tower, or he must design it so as to insure that the natural frequencies of the tower are certain to be different from the expected vibrational frequencies.
Although it is possible to arrange the design so the natural frequencies differ from the expected vibrational frequencies, the result often is that some of the other design objectives must be compromised. Furthermore, a tower design suitable for one windmill must often be completely changed to be adequate for a second windmill, even when the second windmill is smaller.
By and large, these problems are not encountered if a single-leg tower is used. For a given support strength, the single-leg tower is more compliant, and its natural frequencies thus are usually well below the expected vibrational frequencies. Use of the single-leg tower has certain drawbacks, however. For a given strength, it requires more structural material. Also, since the bending moment of the single leg can be very great, its foundation must be considerably more elaborate than the foundation for the multi-leg tower. Also, the single-leg tower is not an open structure like the multi-leg tower, so it provides greater interference with wind flow. This in turn somewhat compromises its main advantage because, although the coupling of vibrational energy into the single-leg tower is generally less efficient, the variations in force that give rise to the vibration tend to be greater.
Nonetheless, a single-leg tower that is adequate for a given windmill can in general be expected to be adequate for another windmill of the same or a smaller mass, and no significant redesign is required. Thus, the single-leg tower has a versatility that prior-art multi-leg towers do not possess.
It is an object of the present invention to achieve some of the versatility of the single-leg design but obtain the considerable benefits of the multi-leg design.